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Full text of "Handbook Of Chemical Engineering - I"

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REFRACTORIES                                       501
dense, whether high in clay or siliceous. This constant is likewise subject to variation both with temperature and structure changes in the material. The value is of the magnitude of 0.0000008 per degree Centigrade, being lower for porous and higher for dense materials.
High-grade flint fireclay products resist temperature changes well but this quality diminishes rapidly with the content of plastic bond clay and the resulting density. As a class these refractories do not resist slagging action well owing to their porosity. 1 his quality is improved either through the increase of plastic clay or by very hard firing.
The mechanical strength of these materials differs widely but as a rule the com-pressive strength does not exceed 1,500 Ib. per square inch.    Bricks of a lower strengt: than 1,000 Ib. per square inch are indicative of inferior bond and hence should not b used.    Little is known concerning the electrical resistance of these refractories a furnace temperatures.
Siliceous-clay Refractories.Owing to the fact that the siliceous clays posses a greater degree of plasticity than the flint-clay mixtures they may be made int bricks in the plastic state by means of the auger machine or piston press. The are fired to about the temperature of cone 8.
From previous considerations it is apparent that this type of refractory canno be us heat-resisting as the previous class. However, for first-class materials the softening point should not be lower than that of cone 28, about 1,635C. or 2,975F. The jMTinanent contraction or expansion upon reheating to 1,400C. is, as a rule, low and should not exceed 0.5 per cent. The porosity may vary between 20-28 per cent. The resistance to compression at temperatures of 1,350C. is high and the contraction should in no case exceed 4 per cent at the temperature given and under a load of 40 Ib. per square inch.
No reliable information is available concerning the thermal properties of these refractories but it is known that their conductivity is somewhat greater than that of the flint-clay refractories. This applies also to the coefficient of thermal expansion. Siliceotm refractories do not resist sudden temperature changes well. They also cannot h(5 expected to resist the action of slags except the latter be siliceous in char-acter. The eompressive strength is about that of the flint-clay brick but the resistance to impact and abrasion is lower.
Plastic-clay Refractories.These materials are usually made into bricks by means of the uugor machine and finally fired to temperature between the softening points of cones 4 to 8. The so-called fusion point of these materials should not be lower than Umt of cone No. 26, about 1,600C. or 2,912F. No load-resisting requirements are made upon refractories of this type but upon reheating to 1,2(.K)( 3. (2,3r>4l<\), maintaining this temperature for 5 hr., they should not show a contraction of more than 1.5 per cent of the original length nor an expansion of more than 1 per cont. These refractories are much denser and harder than those of thcj previous classes.
The porosity may often be as low as 10 per cent. Their thermal properties may vary widely and both the conductivity and expansion are greater than those of the higher grade refractories and often as high as 10 B.t.u. per hour, through 1 sq. ft. and 1 in. thickness, per degree Fahrenheit and at 1,400F. Their resistance to sudden temperature changes is certain to be less than that of the flint-clay refractories. Their resistance to corrosion is limited by the softening temperature. Where the metal or slag is well within the safe limit of the material the resistance is usually good and the shapes used in ladles for handling molten metal are often made from this